Move KCMs in "Workspace Theme" to more appropriate locations

Summary:
This is patch one out of three in support of the new Appearance section layout in Plasma 5.16 decided upon on in T8871: Systematic KCM reorganisation.

In this patch, KCMs are moved out of the Workspace Theme category, and the ordering of some KCMs is changed accordingly. The now-empty category gets deleted in D18417.

Test Plan:

Reviewers: VDG, Plasma, davidedmundson, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, GB_2, abetts, davidedmundson, plasma-devel

Tags: Plasma

Maniphest Tasks: T8871

Differential Revision: https://phabricator.kde.org/D18419